Loop current detector

ABSTRACT

A detector is provided for unidirectional currents flowing in a loop circuit. A dc amplifier coupled to a resistor in the loop provides an output signal proportional to the loop current. A voltage drop across a second resistor in the loop is supplied to the input of a voltage threshold detector and to the output of said amplifier. Means are provided to indicate when said input is less or greater than a reference voltage.

United States Patent 1191 UNITED STATES PATENTS 5/1971 Lord 179/18 FALord Dec. 25, 1973 [54] LOOP CURRENT DETECTOR 2,984,752 5/1961Giacoletto 307/300 3,412,264 11/1968 Preston 307/235 [75] Invent MichaelR 2,767,330 10/1956 Marshall; 307/235 x England 3,156,778 11/1964 Cirone179/18 FA 3,410,961 11/1968 Slana 179/18 FA [73] Ass'gnee' g'gf m x fifig gfi' 3,478,175 11/1969 Herter 179/18 FA 3,525,816 8/1970 Herter179/18 FA [22] Filed: Dec. 6, 1971 Primary Examiner-John W. Huckert [2]]Appl' N 205,352 Assistant Examiner-L. N..Anagnos Related US. ApplicationData Attorney-C. Cornell Remsen, Jr. et a1. [63] Continuation of Ser.No. 10,365, Feb. 11, 1970, abandoned. I 52 ,s 1 307 235 179 1 AA, 179 1EA, A detector is provided for unidirectional currents '179 1 FA flowingin a" loop circuit. A dc amplifier coupled to a 51 Int. Cl. 11041113/18, 1-104 3/06 resistor in the p Prbvides an Output Signal P p [58]Field 61 Search 307/235, 297; tional I9 the 9 current A voltage dropacross a 32'4 140 179 3 FA, 1 EA, 16 73 R 0nd resistor in the loop issupplied to the input of a 1 voltage threshold detector and to theoutput of said [56 R f r Ci amplifier. Means are provided to indicatewhen said input is less or greater than a reference voltage.

6 Claims, 3 Drawing Figures PATENTEDUEB25|9Y5 3.781571 Inventor M. RLord A (tome y y l LOOP CURRENT DETECTOR This isa continuation ofapplicationS er. No. 010,365 filed Feb. 1 l, 1970, now abandoned.

The present invention 'relatesito a detector for unidirectional currentsand in particular toamodification of a circuit described in a.co-pending US. Pat. Application, Ser. No. 775,600now US. Pat.No..3,579,l06.

In the detector of the parent specification unidirectional currentsflowing .in a subscriber loop are detected by coupling to each leg ofthe loop a d.c..amplificr which givesan output current proportional tothe loop current. By combining said output currents, by comparing theresultant .current with a reference currentxand by indicating when theresultant current is less or ,greater than the reference current, theindication is made independent of any longitudinal currents induced.above. In the present invention, a-detector is provided forunidirectional currents flowing in a loop circuit. A resistor isconnected ineach leg of the loop. A d.c. amplifier is coupled to a firstof said resistors to give an output current proportional to said loopcurrent; A voltage threshold detector .is provided. Means are providedto couple the voltage drop across the second resistor to theinput of thevoltage threshold detector and also to couple the output of saidamplifier to the input of the voltage threshold detector. The thresholddetector supplies outputs and to indicate when said input is less orgreater than areference voltage, the indications being independent ofany longitudinal currents flowing in the loop.

The invention will now be described with referenc to the figure of theaccompanying drawing showing in schematic form the detector according toanembodiment of the invention.

In the figure a 2-wire subscriber loop circuit is indicated byconductors .1 and 2 which are connected at the exchange to a battery ofvoltage V via resistors R and R These resistors limit the loop currentin the event of a short circuit and provide a voltage drop proportionalto the current flowing in each leg.

The voltage drop V developed across resistor R by the loop current isapplied via resistor R to the base where K (l /V and is chosen to equall/R I therefore IN- VI lN)/( IN 3) As V, is proportional to the loopcurrent I L the voltage V is also proportional to I so that by choosinga suitable value "of voltage threshold the detector will give an outputwhen I exceeds a predetermined value.

A longitudinal current I, superimposed an 1,, in the direction shownwillincrease the current flowing in leg 1 of the loop and decrease thecurrent flowing in leg 2 of the loop, so that the total voltage drop V Vremains unaffected by I and does not influence the indication of thedetector.

Neglecting the base-emitter voltages and assuming infinite current gain(h of transistor VT, then Where V is a bias applied to the base of thetransistor and R is a resistor connected in series with the emitterelectrode.

.If R is made equal to R then the base-emitter potential of VT andassuming an infi-- I nite h the transistor will be turned off when V, is

electrode of transistor VT which acts as a voltage threshold detector.

The voltage drop V developed across resistor R is applied to the inputof a d.c. amplifier comprising transistor VT The output of thisamplifier is also connected to the input of the threshold detector.

To calculate the voltage V, applied to the input of the thresholddetector, having an input impedance R by the voltage drop V acrossresistor R and bythe output current I, of the d.c. .amplifier thesimplifying assumptions will be made that R, R that is that V, V andthat R R The simplified equivalent network isshown in FIG. 2 in whichthe same designations are used as in FIG. 1.

As R R 'the voltage drop produced in R, by I, can be neglected, so thatthe circuit further simplifies to that shown in FIG..2b.

Designating by I, the input current to the detector we have I nt/ m? 1 1s V1 0 3" nv a rearranging uv 1+ o a)/( nv 3) 11v)/( uv) more negativethan V,, and turned on when V, is less negative than V,,

A feature of the present invention and of the main patent is therelative insensitivity of the circuits to changes of voltage of thebattery supplying the loop current. Since the output current I, of thed.c. amplifier is proportional to the loop current, a given percentagechange of battery voltage will produce approximatelythe same percentagechange of I and'input voltage to the threshold detector. This effect canbe compensated even further by deriving the bias voltage V,, from thesame battery supplying the loop current. This is achieved by connectingthe emitter electrode of VT to a tap of a potential divider connectedacross the battery I claim: 1. A detector for detecting unidirectionalloop currents while providing compensation to avoid measuringlongitudinal currents flowing through a closed circuit 0 formed toinclude first and second legs terminating rend resistor to provide asecond output current proportional to current in the second leg of thecircuit, a voltage threshold detector, means coupling said first outputcurrent from said third resistor and said second output current fromsaid do. amplifier t0 the input terminal of 5 said threshold detector,said threshold detector responding to said first and second outputcurrents to provide an output potential proportional to the loopcurrents flowing in the loop and independent of any longitudinalcurrents flowing in the loop.

2. A detector as claimed in claim 1 in which the direct currentamplifier and the voltage threshold detector each include transistorcircuits.

3. A detector as claimed in claim 1 in which the ratio of the voltagedrop across said first resistor to the output current of the amplifieris equal to the value of said coupling resistor.

4. A detector as claimed in claim 3 in which the amplifier includes atransistor, the input circuit to the transistor includes a biasingresistor of value equal to said third resistor, the former beingconnected in series with the emitter electrode of the transistor.

5. A detector as claimed in claim 4 in which a source of bias potentialis provided in series with the emitter electrode of the transistoracting as a voltage threshold detector.

6. A detector as claimed in claim 5 in which the source of biaspotential to the emitter of the voltage threshold detector is a voltagedrop derived from a potential divider connected across the batterysupplying the loop current.

1. A detector for detecting unidirectional loop currents while provIdingcompensation to avoid measuring longitudinal currents flowing through aclosed circuit formed to include first and second legs terminatingrespectively at ground potential and at d.c. potential, said detectorcomprising a first resistor coupled in the first leg of the circuitbetween a first terminal and ground potential, a second resistor coupledin the second leg of the circuit, a third resistor coupled to the firstresistor via said first terminal to provide a path for a first outputcurrent proportional to current in the first leg of the closed circuit,a d.c. amplifier coupled to the second resistor to provide a secondoutput current proportional to current in the second leg of the circuit,a voltage threshold detector, means coupling said first output currentfrom said third resistor and said second output current from said d.c.amplifier to the input terminal of said threshold detector, saidthreshold detector responding to said first and second output currentsto provide an output potential proportional to the loop currents flowingin the loop and independent of any longitudinal currents flowing in theloop.
 2. A detector as claimed in claim 1 in which the direct currentamplifier and the voltage threshold detector each include transistorcircuits.
 3. A detector as claimed in claim 1 in which the ratio of thevoltage drop across said first resistor to the output current of theamplifier is equal to the value of said coupling resistor.
 4. A detectoras claimed in claim 3 in which the amplifier includes a transistor, theinput circuit to the transistor includes a biasing resistor of valueequal to said third resistor, the former being connected in series withthe emitter electrode of the transistor.
 5. A detector as claimed inclaim 4 in which a source of bias potential is provided in series withthe emitter electrode of the transistor acting as a voltage thresholddetector.
 6. A detector as claimed in claim 5 in which the source ofbias potential to the emitter of the voltage threshold detector is avoltage drop derived from a potential divider connected across thebattery supplying the loop current.